1. Field of the Invention
The present invention relates to a semiconductor light-emitting element, an electrode and a manufacturing method for the element, and a lamp, and more particularly to a semiconductor light-emitting element having an electrode with improved junction and anticorrosion properties, an electrode and a manufacturing method for the element, and a lamp.
Priority is claimed on Japanese Patent Application No. 2008-157248, filed in the Japan Patent Office on Jun. 16, 2008, Japanese Patent Application No. 2008-199802, filed in Japan Patent Office on Aug. 1, 2008, Japanese Patent Application No. 2008-228133, filed in the Japan Patent Office on Sep. 5, 2008, and Japanese Patent Application No. 2009-133177, filed in the Japan Patent Office on Jun. 2, 2009, the contents of which are incorporated herein by reference.
2. Description of Related Art
Recently, as a semiconductor material for a short-wavelength light-emitting element, a GaN-based compound semiconductor has received attention. A GaN-based compound semiconductor is formed on a substrate of various oxides or group III-V compounds including sapphire monocrystals by a thin film-forming means such as a metalorganic chemical vapor deposition (MOCVD) method or a molecular beam epitaxy (MBE) method.
A thin film made of a GaN-based compound semiconductor has the characteristic of small current diffusion in an in-plane direction of the thin film. Further, a p-type GaN-based compound semiconductor has the characteristic of a high resistivity in comparison to an n-type GaN-based compound semiconductor. Due to this, in the case where only a p-type electrode made of metal is laminated on the surface of the p-type semiconductor layer, there is almost no spread of current in the in-plane direction of the p-type semiconductor layer.
In a semiconductor light-emitting element using the GaN-based compound semiconductor as described above, if a laminated semiconductor layer having an LED structure composed of an n-type semiconductor layer, a light-emitting layer, and a p-type semiconductor layer is formed, and a p-type electrode is formed on the p-type semiconductor layer of an uppermost portion, only a portion of the light-emitting layer that is positioned just below the p-type electrode emits light. Accordingly, in order to extract the emitted light, which occurs just below the p-type electrode, out of the semiconductor light-emitting element, it is necessary to make the p-type electrode transmit the emitted light and extract the light by making a p-type electrode having transparency.
As a method of making a p-type electrode having transparency, a method is known that uses a conductive metal oxide such as ITO having transparency or a metal thin film of about several tens of nanometers. For example, PATENT DOCUMENT 1 discloses a method that uses a metal thin film of about several tens of nanometers, and proposes to simultaneously perform acceleration of low resistance of the p-type semiconductor layer and formation of the p-type electrode having transparency and ohmicity by laminating Ni and Au of about several tens of nanometers on the p-type semiconductor layer as the p-type electrode and then performing an alloying treatment through heating under an oxygen atmosphere.
However, the transparent electrode composed of metal oxide such as ITO or the ohmic electrode composed of a metal thin film of about several tens of nanometers has as low a strength as the electrode itself, and thus it is difficult to use the electrode itself as a bonding pad electrode.
In order to improve the strength of the electrode itself, a bonding pad electrode has been used, in which a pad electrode for bonding with a specified thickness is arranged on a p-type electrode, such as a transparent electrode made of metal oxide such as ITO, an ohmic electrode made of a metal thin film of about several tens of nanometers.
However, since this bonding pad electrode is a metal material having a specified thickness, it has no transparency and shields the emitted light that has transmitted the p-type electrode having transparency. As a result, there is a problem that it may not be possible to extract a portion of the emitted light out of the light-emitting element.
In order to solve this problem, for example, PATENT DOCUMENT 2 discloses a method of laminating a bonding pad electrode made of a reflection film, such as Ag, Al, on a p-type electrode. Accordingly, it is possible that the emitted light that has transmitted the p-type electrode is reflected to the inside of the light-emitting element by the bonding pad electrode and the reflected light is extracted from a portion other than a bonding pad electrode forming area to the outside of the light-emitting element.
However, if it is intended to bond a bonding wire or the like with the bonding pad electrode in the case where the metal oxide such as ITO is used as the p-type electrode and the reflection film such as Ag, Al, is used as the bonding pad electrode, the bonding pad electrode is unable to resist the tensile stress during bonding of the bonding wire, and thus the pad electrode may be peeled off.
Also, as the bonding pad electrode is peeled off from the transparent electrode, the yield in manufacturing a lamp that uses such electrodes may deteriorate.
Also, in the semiconductor light-emitting element in the related art, the anticorrosion property is insufficient, and thus there is a demand for improvement of the anticorrosion property.